We have developed assays to detect oxidative lesions in specific genes and to quantitate their formation and repair. Oxidative DNA damage is generated by several different approaches including hydrogen peroxide, acridine orange, X-irradiation, irradiation with methylene blue. The main lesion which we examine is 8-OH guanosine which can be detected by use of the Fpg glycosylase. This enzyme creates strand breaks in DNA at sites of the lesions, and the single stranded DNA can then be resolved on alkaline gels. We find that 8-OH guanosine is rapidly repaired in active genes and in mitochondrial DNA. We have developed an in vitro assay for DNA nicking activity in mitochondrial extracts from rats. We have identified a protein from mitochondria which is able to incise an oligo containing a site specific 8-oxoguanine adduct. Unlike the bacterial enzyme which cleaves both 3' and 5' to the lesion, the mitochondrial enzyme incises exclusively on the 3' side of the lesion. We have finalized our chromatography steps and have begun the physical characterization of the enzyme.